In recent years, high-strengthening and sheet metal thinning of automotive parts have been required. As the steel sheets used for automotive parts have higher strength, press formability decreases, and it becomes more difficult to form the steel sheets into the desired part shape.
To address this issue, some conventional techniques propose performing hot press forming on a blank sheet heated to high temperature to have a desired shape using a tool of press forming, while quenching the blank sheet in the tool of press forming by utilizing heat releasing, to achieve high-strengthening of the hot press formed part. When elements are expressed in “%” herein, this refers to “mass %.”
For example, GB1490535A (PTL 1) proposes a technique for achieving high-strengthening of a formed part by hot pressing a blank sheet (steel sheet) heated to an austenite single phase region and quenching the blank sheet in a tool of press forming simultaneously with the hot press forming.
In addition, JP2011088484A (PTL 2) describes a hot press forming method, in which hot press forming is performed such that a reinforcing steel sheet is overlapped on a steel sheet in need of reinforcement at a portion to be reinforced, in order to achieve high-strengthening of automotive parts by reinforcing the parts only at a specific portion to be reinforced in a more sufficient way while suppressing an increase in the weight of the automotive parts.
However, the techniques proposed in PTLs 1 and 2 have a problem in that heating of a steel sheet to a high temperature around 900° C. before press forming causes oxided scales (iron oxides) on the surface of the steel sheet, and such oxided scales come off from the surface during hot press forming and damages the tool of press forming or the surface of the hot press formed part. Such oxided scales remaining on the surface of the formed part also lead to poor appearance and degraded coating adhesion properties. Accordingly, oxided scales on the surface of the formed part are typically removed by a process such as pickling, shot blasting, or the like. Such processes, however, degrade productivity. Additionally, some parts are required to have high corrosion resistance, such as automotive suspension parts, structural parts of automotive bodies, and the like. However, hot press formed parts manufactured by the methods in PTLs 1 and 2 do not have rust preventive films such as coating layers, and are insufficient in corrosion resistance.
For these reasons, there has been demand for a hot press forming technique that can suppress formation of oxided scales upon heating prior to hot press forming and that can improve the corrosion resistance of hot press formed parts. To meet this demand, other conventional techniques propose coated steel sheets having films such as coating layers on their surfaces, and hot press forming methods using such coated steel sheets.
For example, JP3663145B (PTL 3) describes a method for manufacturing a hot press formed part having a Zn—Fe— or Zn—Fe—Al-based compound provided on a surface thereof and exhibiting good corrosion resistance by performing hot pressing on a coated steel sheet coated with Zn or a Zn-based alloy. However, in a hot press formed part manufactured by the method described in PTL 3, liquid metal embrittlement cracking may be caused by Zn in the coating layer, although formation of oxided scales is suppressed to some extent. Liquid metal embrittlement cracking causes the hot press formed part to suffer performance degradation, such as in fatigue strength, which is problematic.
Accordingly, JP2013184221A (PTL 4) proposes a method for manufacturing a hot press formed part by using a hot press forming object formed from overlapped coated steel sheets, each having a Zn or Zn alloy coating layer formed thereon, the method including: providing protrusions on overlapped coated steel sheets to form a gap of 0.03 mm to 2.0 mm between the overlapped coated steel sheets; and causing Zn present in a liquid phase state at the overlapping portion to evaporate into steam upon heating to suppress liquid metal embrittlement cracking.
In addition, JP2013091099A (PTL 5) describes a method for manufacturing a hot press formed part by using a coated steel sheet having a Zn—Fe-based coating layer formed thereon, in which to suppress liquid metal embrittlement cracking, a coated steel sheet is cooled to a temperature of no higher than the solidification point of the coating layer before subjection to press forming.